Due to the advantages that flexible circuit boards provide, such as light weight, thinness, flexibility, and bendability, flexible circuit boards are widely used in optical modules and, in particular, used for securing and connecting to external elements to realize interconnection between a plurality of external elements. In order to obtain higher bandwidth, high frequency design requires that a base of a metal housing of a TO-CAN (Transistor Outline-Can) be electrically connected to a GND (ground) reference layer of the flexible circuit board by means of soldering. FIG. 1 is a cross-sectional structural diagram illustrating a flexible circuit board secured and connected to an external element in a conventional technology. As illustrated in FIG. 1, a flexible circuit board 9 usually includes, stacked in the following order, a cover film 91, an adhesive layer 92, a signal layer 93, a substrate 94, a ground layer 95, an adhesive layer 96, and a cover film 97. The signal layer 93 and the ground layer 95 are made of copper. In order to make the flexible circuit board 9 be soldered and connected to an external element 8 through a tin solder layer 89, usually the ground layer 95 beneath a securing portion 902 is exposed by means of opening a window in the cover film 97 (i.e., removing a portion of the cover film 97 that covers the securing portion 902 of the flexible circuit board 9). The flexible circuit board 9 also includes a connecting portion 901 which may be bent when the flexible circuit board 9 is being used. Although not labeled in FIG. 1, the flexible circuit board 9 also includes a copper plating layer formed between the substrate 94 and the tin solder layer 89.
However, in a subsequent electroplating process of the flexible circuit board 9, the thickness and hardness of the metal at the securing portion 902 increase as a result of joining the ground layer 95 and the tin solder layer 89. Thus, in the process of bending and using the flexible circuit board 9, it is possible for the ground layer 95 to break along a line “m” (as illustrated in FIG. 1) where the thickness of the metal changes, leading to low structural strength and poor functionality of the flexible circuit board 9 and even a waste of products.